GB2190035A

GB2190035A - Film extrustion die-lip heater

Info

Publication number: GB2190035A
Application number: GB08611188A
Authority: GB
Inventors: Ricardo Pablo Rodriguez; Alfredo Bentivoglio
Original assignee: POLYSYSTEMS MACHINERY MANUFACT
Current assignee: POLYSYSTEMS MACHINERY MANUFACT
Priority date: 1986-05-08
Filing date: 1986-05-08
Publication date: 1987-11-11
Also published as: US4830595A; US4842504A; JPH01502892A; AU7353887A; EP0305372A1; GB8611188D0; WO1987006879A1

Description

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SPECIFICATION

acts to lubricate the flow of resin through the nozzle.

Die-lip heater

5 This invention relatestoa manner of heatingthelip of a die, such as an extrusion die. The invention relates also to the production of plastic film by extrusion.

10 Background to the invention

It is common practice to manufacture plasticfilm in the following manner. First, plastic in molten - i.e. liquid and semi-liquid-form is extruded through an annular nozzle. The resulting tube is subjected to an 15 internal air pressure, which inflates the tube and causes itto balloon outto many times the diameter of the annular nozzle. The inflated tube cools and cures, and is gathered up on rollers.

The economics of the process gives rise to a 20 compromise or trade-off between the quality of the extruded film and the speed at which the film is produced. If the film passes through the nozzle too quickly, the surface texture starts to acquire the so-called "sharkskin" quality.

25 If the width of the nozzle gap is increased, the sharkskin effect can be alleviated. However, ifthe nozzle gap is wide, the film has to be stretched or inflated even moretogetthethickness ofthefilm down to the desired value. The more the tube is 30 inflated, the more likely itisthatthefilm will be uneven as to its thickness and also will have other inferior properties.

The invention is aimed at easing the compromise between on the one hand improving the speed at 35 which plasticfilm with good properties can be produced, and on the other hand avoiding the difficulties of a sharkskin surface, and of avoiding also the difficulties of the variations in thethickness etc. ofthefilm thatfollow from increasing the width 40 of the nozzle.

The prior art

It has been recognized that the sharkskin phenomenon is due to the fact that the extrusion 45 nozzle exerts a frictional drag on the uncured plastic resin passing through the nozzle. The surface of the resin is in contact with the lips of the nozzle and the surface is inevitably therefore travelling at a slower speed than the bulk of the resin. The resin is in the 50 form of a viscous liquid, which has only a relatively small capacity to cope with velocity gradients in the flowing resin.

When these velocity gradients are too large, the shear stresses between the suface and the bulk of the 55 flowing resin become too great. The resulting film has the rough, cloudy surface associated with the sharkskin effect.

It has been recognized thatthe onset of the sharkskin effect can be contained by reducing the 60 friction between the moving resin and the lips of the nozzle. Attempts have been made to heat the lips of the nozzles; in theory, the highertemperature reduces the viscosity of the surface layer of the resin, thus reducing the internal shear stresses. It may be 65 regarded thatthe more-liquid resin atthe surface

General description of the invention

The invention is aimed at heating the lips of the 70 nozzles in such a way thatthe temperature and velocity gradients in the resin are as nearly constant as possible all around the circumference of the nozzle. It is recognized, in the invention, as most importantthatthe lips be heated smoothly and 75 evenly. There should be no local hot-spots or cold-spots around the circumference. It is recognized thatthe tube of film is extremely sensitive to temperature differences around the circumference — even to the extent thatthe lips of the extrusion nozzle 80 should be protected from drafts in the surrounding air.

It is also recognized, in the invention, as very importantthatthe heated lips be somewhat isolated or insulated from the main bulk of the dies. Ifthe 85 overall die temperature were to rise because of the extra heat being supplied to the lips, then again the quality of the film would suffer. It is recognized in the invention that it is possible economically to provide a sufficiently effective degree of insulation between 90 the lips and the bulk of the dies. A preferred manner of arranging the insulation will be described presently.

In the invention, the inner and outer lips of the annular nozzle are provided each with a respective 95 ring of aluminum or other heat conducting material. The ring is let into a respective groove cut in the material of the dies, close to the nozzle lips.

Everything aboutthe ring and groove is aimed at ensuring that all points on the ring are maintained at 100 the same temperature. The ring and groove are continuous and uninterrupted around the circumferences. Neitherthe groove northe ring varies, around the circumference, as to its cross-section. The groove is deep enough thatthe 105 ring within the groove is not exposed to drafts.

Each ring is accompanied by a respective heating means, which may comprise a coil of heater wire. The coil is let into a groove cut, in turn, into the material of the ring.

110 The ring is arranged so as to transfer its heat to the nozzle lips, but so as not to transfer heat to the bulk of the die. This can be achieved by placing those surfaces of the ring that face the nozzle in intimate, heat-transmitting, contact with the material of the 115 walls of the groove, while placing the surfaces of the ring th at f ace a way f ro m th e I i ps, a n d towa rds the bulk of the die, in a spaced apart, insulating, relationship with the walls of the groove.

Using the invention, it has been found possiblefor 120 the nozzle lips to settle to a considerably higher temperature than the bulk of the dies.

In the invention, higher production rates can be achieved than have been possible hitherto. The compromise that affects production speed lies 125 between making the nozzle neithertoo narrow nor too wide, radially. Ifthe nozzle is too narrow, high speed production leadsto the sharkskin effect. Ifthe nozzle is too wide, the properties of the film startto deteriorate. The invention eases the compromise. 130 It is recognized in the invention that previous

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proposals for heating the nozzle lips have been less successful because not enough attention was paid to keeping the temperature even all aroundthe circumference of the nozzle.

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Detailed description of a preferred embodiment of the invention:

Inthedrawings:-

Figure 1 is a cross-sectional view of an extrusion 10 die which embodiesthe invention;

Figure 2 is a close-up view of part of the extrusion nozzle of the die of Figure 1;

Figures 3 and 4 are pictorial view of components of the die of Figure 1.

15 In the drawings, the physical features of the components have been shown somewhat exaggerated forthe sake of clarity.

The die set shown in Figure 1 is conventional apart from the heaters. Molten resin is fed underpressure 20 through a conduit 1, through a siral channel 2, and into a collecting chamber 3. The resin then passes through the annular nozzle 4.

Air is blown into the centre of the annulus, through the passage 5, and the air inflates the extruded tube. 25 The plastic cools and cures in a few seconds. The tube is collapsed between pinch rollers, and coiled onto a drum, or collected in some other suitable manner. The tube may be slit and sold as plasticfilm or may remain in tubularform for manufacture into 30 plastic bags.

The nozzle 4 has an inner lip 6, which isformedon an inner die 60, and an outer lip 8, which isformed on an outer die 80.

Formed in each of the dies 60,80 are respective 35 heater grooves 61,81. In these grooves are fitted respective aluminum heater rings 62,82. The rings 62,82 each have a respective coil-receiving groove 63,83, into which a respective coil of heater-wire 64, 84 is inserted.

40 The surfaces 65,85 of the rings 62,82 which face towards the nozzle are arranged to fit intimately against the complementary surfaces of thegrooves 61,81, over the full area of these inter-engaging surfaces. This ensures a good heattransfer between 45 the ings 62,82 and the nozzle lips 6,8.

The surfaces 66,86,67,87 of the rings 62,82 which face away from the nozzle 4 are arranged to be spaced away from the complementary surfaces of the grooves 61,81. The purpose of this is that only a 50 small amount of heat is transferred between the rings 62,82 and the bulk of the dies 60,80.

It is important, though, thatsmall lands 68,88 be left on the surfaces 66,86 of the rings 62,82. These lands 68,88 ensure thatthe rings 62,82 are a tightfit 55 in the grooves 61,81.

It is important forthe rings to be a tightfit in the grooves because the rate of expansion of the aluminum of the rings is different from that of the steel of the dies. Ifthe aluminum were nottightly 60 constrained, the intimate and heat-transferring contact might be lost, i.e. the intimate contact that exists between the surfaces 65,85 of the rings 62,82 and the complementary surfaces of the grooves 61, 81. This intimate contact might tend to work loose 65 after a number of cycles of expansion and contraction. The tightfit ensures thatthe surfaces 65, 85 are always pressed tightly against the sides of the grooves and therefore able to transmit heat to the lips 6,8 of the nozzle 4.

Small lands 69,89 have also been left on the bottom surfaces 67,87 of the rings. These lands ensure that there is little heat conducted from the rings to the dies through the bottom surfaces. The lands 69,89 could in fact be omitted, and the rings could be left quite free of all contact with the bottom of the groove. The lands 69,89 do however serve to define the point at which the rings 62,82 reach bottom in the grooves 61,81.

The coils 64,84 comprise each a few turns of electrical resistance wire. Again, every attempt is made to keep the heater rings, and therefore the nozzle lips, as even in temperature as possible around the circumference. The coils may be embedded in a refractory cement within the grooves 63,83. It will be noted thatthe cement need not be compatible with the alumiunum as regards expansion rate, because both the rings and the coils are very tightly constrained by the presence of the comparatively massive dies against movement relative to each other.

Claims

1. Apparatus which issuitableforusein producing plasticfilm by extrusion; characterised:-

in thatthe apparatus includes an inner die (60) and an outer die (80);

in thatthe dies have complementary surfaces which comprise the respective inner (6) and outer (8) lips of an annular nozzle (4);

in thatthe apparatus includes respective innerand outer lip heaters;

in that each heater includes a respective annular ring (62,82), and a respective means (64,84) for heating the ring;

in that each ring (62,82) includes a respective means for ensuring that substantially all locations around the annulus of the ring are at substantially the same temperature; in thatthe apparatus includes means for ensuring that heat from each ring (62,82) is transferred easily and evenly to the respective lip (6,8)ofthenozzle(4);

in thatthe apparatus includes means for insulating the respective bulks of the two dies (60,80) against heattransferfrom the respective ring;

and in thatthe apparatus includes respective means for effectively isolating each ring (62,82) againstthe impingement against the ring of drafts in the surrounding air.

2. Apparatus of claim 1, where the rings (62,82) are received in respective deep grooves (61,81) formed one in each of the dies (60,80).

3. Apparatus of claim 1, where each ring (62,82) isformed from a respective single piece of material, which extends continuously and without interruption around the annulus of the ring;

and where the form of the ring in cross-section taken at one radius of the ring is in substance identical to the form of that ring in cross-section taken at every other radius.

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4. Apparatus of claim 3, where each ring (62,82) ismadeofaluminum.

5. Apparatus of claim 2, where each ring (62,82) is a tight fit radially in its respective groove (61,81).

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6. Apparatusof claim 5,whereeach groove(61, 81) has a respective f i rst su rface which faces towards the nozzle (4) and second surfaces which face in directions other than towards the nozzle;

and where the respective ring (62,82) is so shaped 10 thatthe material (65,85) of the ring lies in intimate contact with substantially the whole of the area of the first surface of the groove, and is so shaped thatthe material (66,67,86,87) of the ring is spaced away from most of the area of the second surfaces of the 15 groove.

7. Apparatusof claim 1,wherethe lip heaters also include espective heating elements;

where each element comprises a length of electrical heating wire wound into a coil (64,84); 20 where the coil is disposed around the annulus of the respective ring (62,82);

and where the coil ofwireisletintoa wire-containing groove (63,83) in the ring.

8. Apparatus which is suitable for use in

25 producing plasticfilm by extrusion, characterised:-in thatthe apparatus includes an inner die (60) and an outer die (80);

in thatthe dies have complementary surfaces (6,8) which comprise the inner and outer lips of an 30 annular nozzle (4);

in thatthe apparatus includes respective inner and outer lip heaters;

in that each heater includes a respective annular ring (62,82) and a respective means for heating the 35 ring (64,84);

in thatthe rings are received in respective deep grooves (61,81) formed one in each of the dies;

in that each ring is formed from a respective single piece of aluminum, which extends continuously and 40 without interruption around the annulus of the ring; in thattheform of the ring (62,82) in cross-section taken atone radius of the ring is in substance identical to the form of that ring in cross-section taken at every other radius;

45 in that each ring (62,82) is a tightfit radially in its respective g roove (61,81);

in that each groove (61,81) has a respective first surface which faces towards the respective nozzle lip (6,8) and second surfaces which face in directions 50 otherthantowardsthe nozzle lip;

in thatthe respective ring is so shaped thatthe material (65,85) of the ring lies in intimate contact with substantially the whole of the area of thefirst surface of the groove, and is so shaped thatthe 55 material (66,67,86,87) of the ring is spaced away from most of the area of the second surfaces of the groove;

in thatthe lip heaters also include respective heating elements;

60 in that each element comprises a respective length of electrical heating wire wound into a coil (64,84);

in thatthe coil is disposed around the annulus of the respective ring;

and in thatthe coil of wire is let into a 65 wire-containing groove (63,83) in the ring.

9. Apparatusfor heating the lip of a die, characterised;

in thatthe apparatus includes a heater; in thatthe heater is located within the bulk of the 70 die;

in thatthe heater is located nearthe lip; in thatthe apparatus includes a means for equalising the temperature of the lip at all points around the lip;

75 in thatthe apparatus includes a means for ensuring that heat from the heater is transferred easilyan evenly to the lip;

and in thatthe apparatus includes a meansfor insulating the bulk of the diefrom the heater.

Printed for Her Majesty's Stationery Office by Croydon Printing Company (UK) Ltd, 9/87, D8991685.

Published by The Patent Office, 25Southampton Buildings, London, WC2A1AY, from which copies may be obtained.